The present invention relates to a method for drying a paper web in which the paper web is supported by a drying wire without substantially long open draws of the web over the length of the portion of the web that is being dried.
The invention also relates to a drying module for the dryer section of a paper machine, which is intended in particular for use in dryer sections of high-speed paper machines wherein the web running speed is from about 25 meters per second to about 40 meters per second. The drying modules include a drying-wire loop guided by guide rolls, drying cylinders, and at least one reversing suction roll.
The invention further relates to a dryer section which includes the drying modules mentioned above.
The invention also relates to various hybrid dryers including the above drying modules and prior art groups of drying cylinders.
The highest web speeds in paper machines are currently already in a range of about 25 meters per second. In the near future, a web speed range above 25 m/s to about 40 m/s will be used. At these speeds, a bottle-neck in the runnability of a paper machine will be the dryer section whose length with the use of the prior art multi-cylinder dryers would also be intolerably long. For example, if it is considered that an existing multi-cylinder dryer were operated at a web speed of about 40 m/s, it would comprise about 70 drying cylinders, and its length in the machine would be about 180 m. In such a case, the dryer would comprise about 20 different wire groups and a corresponding number of group-gap draws. It can be assumed that, in a speed range of from about 3.0 m/s to about 40 m/s, the runnability of normal prior art multi-cylinder dryers would not be even nearly satisfactory, but there would be a great number of web breaks, which lowers the efficiency of the paper machine.
In the speed range of from about 30 m/s to about 40 m/s and at higher web running speeds, the prior art multi-cylinder dryers would become uneconomical to operate because the investment cost of an excessively long paper machine hall would become unduly high. It can be estimated that the cost of a paper machine hall is currently typically about 1 million FIM per meter (about $175,000 per meter) in the machine direction.
In a paper machine hall, space is usually available in the direction of height (usually the z-direction), and so it has been suggested that the cylinders in a multi-cylinder dryer be arranged in vertical stacks. However, in such an arrangement, especially when used in paper machines having high web running speeds, the problems of runnability and broke removal are emphasized and are likely to be very difficult to solve in the speed range of from about 30 m/s to about 40 m/s. With respect to the prior art showing such a vertical arrangement, reference is made to the assignee's Finnish Patent Application No. 890786 (corresponding to U.S. Pat. No. 4,972,608, the specification of which is incorporated by reference herein).
One parameter that illustrates the drying capacity of prior art multi-cylinder dryers is the amount of water evaporated in the dryer section per unit of length and width, i.e., per floor area covered by the web to be dried, in a unit of time. In the prior art multi-cylinder dryers, this parameter is typically in a range of from about 50 kilograms of H.sub.2 O per square meter in an hour to about 80 kilograms of H.sub.2 O per square meter in an hour (kg H.sub.2 O/m.sup.2 /h).
It is known in the prior art to use various blowing-on/blowing-through units for evaporation drying of the paper web. These units have been used in particular for drying tissue paper. With respect to the prior art showing such units, reference is made by way of example to the following patents: U.S. Pat. Nos. 3,301,746, 3,418,723, 3,447,247, 3,541,697, 3,956,832, 4,033,048, Finnish Patent No. FI 57,457 (corresponding to U.S. Pat. No. 4,064,637 and Swedish Patent No. SE 7503134-4), and Finnish Patent No. FI 87,669 (corresponding to Finnish Patent Application No. 920942 and U.S. patent application Ser. No. 08/025,191 filed Mar. 2, 1993, the specification of which is incorporated by reference herein).
One notable patent among the patents listed above is U.S. Pat. No. 4,033,048 (assigned to J. M. Voith) which describes a paper web dryer. However, the dryer described in this patent is not suitable for use in paper machines where the speeds of the web is high, e.g., greater than about 25 m/s, and to which the present invention is directed. Therefore, the paper machine described in this reference is clearly not suitable for specific web running speeds in a range from about 30 m/s to about 40 m/s or even higher.
In this respect and in other respects, the device described in the '048 patent has several drawbacks as follows. Briefly, in the device of the '048 patent, a suction box is arranged inside a support-fabric loop. By means of the suction box, both a large suction roll and a pocket placed underneath the suction roll and between outside heated rolls are subjected to negative pressure. However, a problem arises in connection with the lateral seals through which significant amounts of air leak. The leakage air produces a strong air current in the transverse direction of the machine in the lateral areas of the web. This air current deteriorates the stable run of the web through the dryer and, consequently, the runnability and the efficiency of the whole machine. Owing to the large amount of leakage air, the subjecting of the pocket and the roll to the level of negative pressure that is necessary at high speeds in order to ensure a stable run of the web, requires large air ducts and blowers and, therefore, consumes a lot of energy. This is a significant drawback which curtails the effectiveness and use of the device of the '048 patent at high web running speeds.